Regional myocardial blood flow was measured using radioactive microspheres during partial coronary occlusion in dogs. Measurements were made before, during, and after a variable occlusion which maintained a constant distal coronary pressure of 40 to 45 mm. Hg. A second group of dogs was studied at 5, 30, 90, and 180 minutes after an occlusion which maintained a constant proximal circumflex coronary artery resistance for three hours.